The recommendations in this article are admittedly biased towards fuses, but for what EPG believes to be valid reasons. Fuses are not always the correct choice for motor protection. However, it has been our experience that fuses are the best choice for the products we are manufacturing. When designing our products we strive to provide a product that will be safe, reliable, and cost effective.
EPG uses current limiting fuses which have a 200,000 Amp interrupting capacity. The typical fuse size used on our applications is smaller than 30 amps, with 200,000 Amps of prospective short circuit current. The current limiting fuses that we use limit the RMS let-thru current to less than 4,000 Amps and the peak let-thru current to less than 8,000 Amps, greatly decreasing the risk to personnel and equipment.
NOTE: When evaluating prospective short circuit current in multiple motor applications you must consider the contribution of all operating motors to the short circuit current in addition to the utility.
Overcurrent protective devices that trip are often reset without first investigating to find the cause of the fault. Electromechanical devices that have opened high level faults may not have the reserve capacity to open at a 2nd or 3rd fault safely. (When a fuse opens it is replaced with a new fuse, the protection is not degraded by previous faults.)
The use of fuses can reduce the dangers of Arc Flash. Arc Flash can be described as an explosion of energy with the potential to cause severe injury and damage to equipment. The costs of medical treatment and equipment repair or replacement due to these incidents can be great. It is more important than ever that employers be diligent in their effort to provide a safe working environment.
A fuse has no moving parts to wear or become contaminated by dust, oil, or corrosion. The speed of response of a fuse will not slow down as the fuse ages. In other words, the fuse’s ability to provide protection is not adversely effected by the passage of time.
Fuses may be coordinated to provide selectivity without compromising short circuit protection; something that is very difficult to do with circuit breakers.
You do not pay a premium for high-interrupting capacity. The fuses we use are rated 200,000 A.I.C. or greater. Devices with lower interrupting ratings are often rendered obsolete by increases in available fault current. Costly system upgrades may be required to maintain system safety.
EPG designs are Type 2 coordinated protection. If a fault should occur, only the zone containing the fault will be effected. The rest of the system will continue to function. Type 2 protection, as a result of the use of current limiting fuses, assures that system components are not subjected to damaging fault currents, minimizing repair costs.
The use of current limiting fuses with high interrupting capacities often allows the use of smaller, lower ampacity disconnects. This also allows the use of the smaller IEC motor starters. Both in turn result in lower initial costs as well as repair costs.
The video clips below help illustrate the power of Arc Flash and the benefits of using Fuses vs. Circuit Breakers.
NEMA Starter Test
IEC Starter Test
*Video clips courtesy of Cooper-Bussmann.
Learn more about the anatomy of Arc Flash and NEC Article 409 regarding industrial control panels in EPG's Summer 2006 newsletter.